1. Field of the Invention
The present invention relates to a sheet feeding device which is preferably utilized in an image forming apparatus such as a document copying machine, a laser beam printer, etc., and more particularly relates to an improvement in a separation mechanism to separate sheets of paper which are sent out from a paper pile section.
2. Description of the Prior Art
It is well known to the public that this kind of a sheet feeding device is of such a type as is provided with a feeding roller to transfer sheets of paper and a pad or a separation roller which is alternately engaged to the feeding roller in the axial direction and which can separate sheets of paper therebetween as a separation mechanism to separate sheets of paper.
There are two types of the above separation mechanism, one of which is such a type that the feeding roller and the pad or the separating roller are brought into contact with each other, and the other of which is a non-contact type as disclosed in the Japanese Utility Model Laid-Open No. Sho-63-45732 and the Japanese Laid-Open No. Sho-63-48744. And both of these types have advantages and disadvantages in the sheets separating capacity and the sheet feeding capacity pertaining to presence of traces of rolls on the sheet.
The sheet feeding device 1' shown in FIG. 5 and FIG. 6 is attached to a document copying machine provided with a double-side copying feature. The sheet feeding device 1' comprises a guide plate 4 as a guiding member which is fixed at the side of a document copying machine along with the sheet feeding direction (arrow "S") and which guides sheets of paper 3 which are in stock on the intermediate tray 2 after an image has been formed on a single side thereof from the intermediate tray 2 in the sheet feeding direction by means of a transfer roller 7 at the side of the document copying machine, a feeding roller 5 of which axis 8 is arranged at the side of a document copying machine with a right angle to the sheet feeding direction and by which sheets of paper 3 can be sent in the feeding direction, and a pad 6' made of rubber having elasticity, which is provided in parallelism with the axis 8 at the side of a document copying machine and can separate sheets of paper 3 sheet by sheet between the pad 6' and the feeding roller 5, thereby causing the sheets of paper 3 to be separated sheet by sheet and to be sent for forming an image on the other side thereof.
The feeding roller 5 and the pad 6' are engaged alternately in non-contact with each other in the axial direction thereof and respective engaging portions thereof constitute a separation portion 9. The separation portion 9 are so set that they can be located at the position on a line "C" can pass through a minimum clearance portion formed between the feeding roller 5 and the guide plate 4 and can intercross at right angles with the guiding surface "l" of the guide plate 4.
Such a sheet feeding device 1' is so composed that sheets of paper 3 can be separated sheet by sheet and can be sent by utilization of the difference of friction forces Frc, Fcc, and Fcp which occur between the feeding roller 5 and sheets of paper 3, between mutual sheets of paper 3 and between sheets of paper 3 and the pad 6' when a sheet of paper 3 passes through the separation portion 9 with the sheet of paper 3 made wavelike in the axial direction. Hence, the strength of the friction forces has been adjusted in the order to Frc&gt;Fcp&gt;Fcc.
Therefore, even though a plurality of sheets of paper 3 are carried in between the feeding roller 5 and the pad 6', only a sheet of paper 3 at the uppermost layer which is in contact with the feeding roller 5 is separated by the difference of the friction forces and is transferred in the separation portion 9. The other sheets of paper 3 lower than the second sheet of paper 3 can be retained by the inclined face of the pad 6', which is inclined from the guide plate 4 toward the feeding roller 5 at the separation portion 9 or beforehand thereof.
On the other hand, as an intermediate tray 2 which has a large stock capacity is needed by influence of recent mass information tendency, an intermediate tray 2 of which height "T" of sheets accommodation is high has been employed. Thereby, the number of sheets of paper 3 which may be carried in the separation portion 9 simultaneously has been diversified.
For instance, as shown in FIG. 7(a), in the case one or two sheets of paper 3 are carried in, the leading edge of the sheets of paper 3 is bent downwards and is brought into contact with the guide plate 4 and the leading side from the portion "d" is bent in the feeding direction (an arrow "S").
Next, the leading edge of sheets of paper 3 is guided along with the guiding surface "l" and is carried in the separation portion 9 where they can be separated sheet by sheet.
Hence, as shown in FIG. 7(b), in the case that a great deal of sheets of paper 3 are transferred from the intermediate tray 2 toward the separation portion 9 at a time, the leading edge of sheets of paper 3 is brought into contact with the face 6'a of the pad 6' and is made just like being jammed before the pad 6 in the sheet feeding direction. Under such a condition, in the case that the engaging width "a" of the separation portion 9 formed between the feeding roller 5 and the pad 6' is large, only the sheet of paper 3 at the uppermost layer is not further sent in beyond the point "Z" of intersection where the feeding roller 5 and the pad 6' intercross when being observed from the side thereof. Contrarily, in the case that the engaging width thereof is small, there may occur such a problem that a number of sheets of paper 3 are sent in toward the separation portion 9 at a time.
Especially, the main reason why a sheet of paper 3 is not further sent in from the above point "Z" of intersection at the edge at the upstream side in the sheet feeding direction of the separation portion 9 is further located at the upstream side in the sheet feeding direction than the minimum clearance portion located on the line "C" between the feeding roller 5 and the guide plate 4. Namely, as the clearance between the feeding roller 5 and the guide plate 4 gets more narrow toward the downstream side in the sheet feeding direction, the leading edge of a sheet of paper 3 which is in a jammed condition before the face 6'a of the pad 6' loses the degree of freedom thereof more as it goes further in the sheet feeding direction.